The invention relates to the field of multiple-gate field effect transistor (FET) and method to fabricate the same, and in particular to fabricating strained-Si multiple-gate FET structures.
One of the primary challenges of very large scale integration (VLSI) has been the integration of an ever-increasing number of MOSFET devices within a single chip. This was achieved mainly in the prior art by scaling down the MOSFET device feature size without excessive short-channel effects. When the device becomes smaller and smaller, short-channel effects, which are caused by the two-dimensional electrostatic charge sharing between the gate and the source/drain diffusion regions, become a serious issue.
To scale down a MOSFET feature size without excessive short-channel effects, multiple-gate MOSFET structures have been developed. Yan, et al., “Scaling the Si MOSFET: From bulk to SOI to bulk”, IEEE Trans. Elect. Dev., Vol. 39, p. 1704, July 1992, have shown that to reduce short-channel effects for 0.05 μm MOSFETs, it is important to have a backside-conducting layer present in the structure that screens the drain field away from the channel. The Yan, et al. results show that double-gated MOSFETs, or MOSFETs with a top gate and a backside ground plane, or in general multiple-gate device, are more immune to short-channel effects and hence can be scaled to shorter dimensions than conventional single-gated MOSFETs.
In all the multiple-gate MOSFETs, there are at least two gates which are opposite to each other, one on each side of the channel. The two gates are electrically connected so that they serve to modulate the channel. Short-channel effects are greatly suppressed in such a structure because the two gates very effectively terminate the drain field line preventing the drain potential from being felt at the source end of the channel. Consequently, the two-dimensional electrostatic charge sharing between the gate and the source/drain diffusion regions in a multiple-gate MOSFET is much smaller than that of a conventional single-gated structure of the same channel length.
Moreover, multiple-gate MOSFET can provide significant performance advantages over the single gate devices. Various multiple-gate processes have been proposed. One promising approach is the FinFET technology, where silicon “fins” are defined on an insulator substrate, and two gates are made on the both sidewall of the fin. High performance FinFET CMOS has been designed using the state of art industry settings (See Y-K Choi, et al “Nanoscale CMOS spacer FinFET for the Terabit era,” IEEE, Electron Device Letters, Vol. 23, No. 1, pp 23-27, 2002).
Another multiple-gate technology is formed using the Tri-Gate CMOS technology, which utilizes the additional top surface besides the two sidewalls (See B. Doyle, et al. “Tri-Gate fully-depleted CMOS transistors: fabrication, design, and layout,” IEEE VLSI Symposium, 2003). Other multiple-gate technology includes Ω-gate technology, gate-all-around technology, etc.
To implement the multiple-gate technology, conventional thin film SOI wafers are used. Since the multiple-gate devices have very small channel dimension, typically less than 100 nm, thus the non-uniformity of the Si film thickness across a SOI wafer will affect the device performance significantly. Thus, the multiple-gate technology requires very good film thickness uniformity. However, the conventional methods for SOI substrate production typically involve costly processes such as high dose ion implantation or wafer bonding. In addition, the SOI substrate produced by conventional methods has limitations in film thickness range and thickness uniformity. There is also no well-established method to produce strained-Si multiple-gate device in the prior art. A strained-Si multiple-gate device is able to combine the benefits of multiple-gate technology and high electron and hole mobility. Strained-Si is shown to enhance electron and hole mobility significantly.